1. Field of the Invention
This invention is a sonic method and means for locating the damage to a liquid cargo tank in a tanker ship that occurred due to collision or impact with an obstruction in the water. This disclosure is related to U.S. patent application No. 07/479,259, filed 02/13/90, and now U.S. Pat. No. 4,960,347, for a Ship-borne Emergency Oil Containment System and Method.
2. Discussion of the Prior Art
In the above-cited disclosure, emphasis was placed upon containing the oil spilled from a damaged tanker ship at the source, namely at the ship itself, rather than letting the spillage spread around over the sea to befoul the environment. In that method, a ruptured cargo tank is emptied into a normally-empty emergency holding tank so that the oil will not get into the water in the first place. Additionally, the ship's crew deploy a ship-borne barrier curtain around the ship, or at least around the damaged tank, to prevent further dispersal of whatever oil that may have leaked out of the ship when the damage first occurred.
Large tankers, on the order of a quarter mile long, are fairly common. On a ship that large, there is a very real problem in timely identifying which one of the many tanks is affected by an impact with an underwater obstacle. Tankers typically have a draft of about 60 feet. By the time the oil plume from the stove-in tank bubbles to the surface much oil will have already escaped before remedial action can be taken. Further, in the presence of a modest ocean current, the plume is carried down-stream so that a punctured cargo tank cannot be accurately spotted by simple visual inspection from the deck.
It would be useful to provide instrumental means within each tank of a tanker for positively identifying a damaged tank. Not only could such a means be used to identify a tank that has actually been cut open to the sea, it could also be used to identify a tank that has been merely bent out of shape and weakened by an impact so that the tank can repaired at the next post-of-call.
U.S. Pat. Nos. 4,412,501 and 4,301,006, both entitled Ship-borne Oil Dispersant Procedure and Apparatus, and filed respectively 06/05/81 and 06/04/79 describe a method and means for dumping an oil dispersant into a ruptured cargo tank. The inventor teaches three means for damage detection: (1) An oil-level gauge for monitoring the oil level in a tank; (2) A pressure detector for detecting a drop in pressure in a tank; and (3) A wall deformation detector, composed of a network of tension wires lining the inside wall of a tank, that upon wall-rupture would send a signal to release the dispersant.
When a tank is ruptured, a good bit of oil is released but at the same time sea water necessarily invades the tank to replace the missing oil. See for example, U.S. Pat. No. 4,241,683 for a Liquid Cargo Tank Construction. An oil level gauge would not be very diagnostic if the oil is replaced by water so that the tank remains full of liquid despite the oil leakage. Similarly, for the pressure detector. The wall deformation detector, including its permanently-mounted network of tension wires, is rather complicated to install and could easily be damaged and rendered ineffective during routine maintenance and tank-cleaning operations. Of course, it would also be hazardous to workers if they needed to physically enter an oil tank to repair the tension wires.
U.S. Pat. No. 4,249,834, issued 02/10/81, discloses a ship having a plurality of relatively small cargo tanks. The concept is that if one of the tanks is slit open, not much oil will escape because the tank does not have a very large capacity. The patentee also teaches use of liquid-level gauges coupled to a display to show which tank is ruptured. Use of liquid-level gauges is ineffective for the reasons stated supra.